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The Swiss-made plane built for the first round-the-world solar flight has wings longer than a Boeing 747 jumbo jet yet weighs only about as much as a large car.

The Solar Impulse 2, unveiled to the world on Wednesday at Switzerland's Payerne air force base, is a bigger and better version of the single-seater prototype that first took flight five years ago.

The original plane demonstrated that a solar-powered plane can fly through the night, hop from Europe to Africa and cross the width of the United States.

But its successor needs to be able to stay in the air far longer, because the pilots expect the lumbering aircraft to take at least five days and five nights to cross the Pacific and Atlantic oceans on its journey around the globe next year.

The new version can theoretically stay airborne indefinitely, according to Bertrand Piccard and Andre Borschberg, who founded the Solar Impulse project over a decade ago. Piccard and Borschberg, who will alternately pilot the plane, admit that now they are the weakest link.

To help them, the plane has an autopilot function, a toilet, a comfortable business-class-style seat and enough space in the ergonomic cockpit for the pilot to lie down and either exercise a little or get some rest.

"I mean, the airplane can fly a month. The question is, what can the pilot do?" Borschberg asked. "So we have a sustainable airplane in terms of energy; we need to develop a sustainable pilot."

American businessman and adventurer Steve Fossett completed the longest non-stop flight in aviation history in 2006, flying 42,469 kilometres in about 76 hours but stopping early because of mechanical problems.

Compared to its predecessor, Solar Impulse 2 has better batteries for storing energy soaked up from the sun by the roughly 17,200 solar cells that cover the massive wings, which at 72 metres are equal to those of the largest passenger planes.

The wingspan, in fact, is eight metres longer than the first prototype - longer even than the wings of a Boeing 747 - but the entire plane still weighs only 2.3 tonnes, about the same as a family car. To maintain its weight budget, the materials in the updated plane are lighter than before, and it has more efficient electric motors.

That's important because while the journey will be broken up into several stages, the aircraft's maximum speed of 140 km/h means it will have to stay in the air for several days in a row during the long transoceanic legs.

"I think we're going to be in this cockpit being aware of the privilege it is to fly in the first and only airplane that can stay in the air forever," Piccard said.

Borschberg said the trip next year would take about 20 flying days, spread over three months. The pilots said they wanted to unveil the plane now because they had just finished building it and would test it during May and June.

The first plane needed perfect weather each day to recharge the battery, and it was smaller and not built to be as trouble-free.

The new plane can cross small cloud layers, and "if it's partly cloudy during the day we can cope with that as well", Borschberg said.

The solitary nature of the flight could be a problem. But adding a second seat would have meant adding too much weight to the plane because another parachute and more oxygen, water and food would also have been needed.